In a combined x-ray diffraction and density functional theory based study the bonding properties of several halogenide boracites have been investigated. We show that DFT calculations reliably reproduce the small spontaneous deformations of non-cubic boracites and that they therefore can be used to predict pressure-induced structural changes. Specifically, we predict the compressibility and the pressure-induced deformation of Mg-Cl boracite. Earlier findings concerning an unusual bond length in Co-Cl boracite are challenged. The arrangement of the halogenide ions and metal cations along chains running through the structure is rationalized in terms of electronegativity differences. We also show that the substitution of the halogenide or metal ions has no significant effect on the B-O bonds.
SummaryCrystals of halogen manganese boracite, the mineral Mg 3 B 7 O 13 Cl, which is currently found in bedded sedimentary deposits of anhydrite, gypsum and halite, have been grown by chemical transport reactions and were examined by polarizing light microscopy and scanning electron microscopy. For both methods the same as-grown samples were used without having to metallize the crystal faces. Comparative electron microscope images were useful not only for observing the charging mechanism of an insulating sample bombarded by an electron beam but also for revealing the domain structure of these ferroelectric/ ferroelastic materials previously observed between the crossed polars of a light microscope. EDS qualitative analysis of the crystal faces was performed for the three compositions under study, i.e.
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